This invention relates to electroluminescent phosphors and more particularly to electroluminescent phosphors that have been treated to be moisture resistant. More particularly, this invention relates to electroluminescent phosphors having greatly reduced moisture absorption and greatly increased life and efficacy.
Treated phosphors are known from U.S. Pat. Nos. 4,585,673; 4,825,124; 5,080,928; 5,118,529; 5,156,885; 5,220,243; 5,244,750; and 5,418,062. It is known from some of the just-mentioned patents that a coating precursor and oxygen can be used to apply a protective coating. See, for example, U.S. Pat. Nos. 5,244,750 and 4,585,673. The treatment processes in several of the others of these patents employ chemical vapor deposition to apply a protective coating by hydrolysis. It also has been reported that chemical vapor deposition, at atmospheric pressure, can be used to deposit thin films of aluminum nitride coatings from hexakis(dimethylamido)dialuminum and anhydrous ammonia precursors upon silicon, vitreous carbon and glass substrates. See, for example, xe2x80x9cAtmospheric pressure chemical vapor deposition of aluminum nitride films at 200-250xc2x0 C.xe2x80x9d, Gordon, et al., Journal Material Resources, Vol. 6, No. 1, January 1991; and xe2x80x9cChemical vapor deposition of aluminum nitride thin filmsxe2x80x9d, Gordon, et al., Journal Material Resources, Vol. 7, No. 7, July 1992. See, also, U.S. Pat. Nos. 5,139,825 and 5,178,911, Gordon, which also disclose transition metal nitrides and other metallic nitrides such as gallium and tin, respectively. U.S. Pat. No. 5,856,009 discloses a high temperature process (i.e., 300 to 700xc2x0 C.) for applying a silicon nitride coating over a previously applied heat resistant coating on phosphor particles. U.S. patent application Ser. No. 09/175,787, filed 10/20/98 (incorporated herein by reference) and which claims priority from Provisional Application Ser. No. 60/072,510, filed Jan 12, 1998, discloses a nitride coating process using a highly reactive hexakis(dimethylamido)dialuminum that has been difficult to scale up to commercial quantities. It would be an advance in the art to provide a process for providing moisture resistant electroluminescent phosphors. It would be a further advance if that process operated in the absence of water or water vapor. It would be a further advance in the art to increase the efficacy and the life of such phosphors manufactured by such a process. It would be a still further advance in the art to provide a process that did not rely upon oxygen. It would be a still further advance in the art to provide an electroluminescent phosphor with a non-oxide coating such, for example, as a metallic nitride coating that is applied directly to the phosphor particles at a low temperature, i.e., about 100xc2x0 C., so that the phosphor performance is not degraded. It would be a still further advance in the art to provide a process employing highly reactive materials that can yield commercial quantities of coated phosphor.
It is, therefore, an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to enhance the operation of moisture-resistant phosphors.
Yet another object of the invention is the provision of a method for providing moisture resistant phosphors that does not employ water or water vapor, or oxygen.
Still another object is the provision of a method and apparatus for providing commercial quantities of nitride coated phosphors which method and apparatus employ highly reactive materials.
These objects are accomplished, in one aspect of the invention, by the provision of a phosphor particle having thereon a coating of a metallic nitride. The coating may be conformal to the particle surface. By conformal is meant a coating that follows the surface contours of the individual particles.
The objects additionally are accomplished by a process of preparing moisture resistant particles of electroluminescent phosphor, comprising the steps of: introducing an inert gas into a reaction vessel that is charged with phosphor particles; heating the reaction vessel to a reaction temperature; introducing a nitride coating precursor into the reaction vessel in a manner to avoid restrictive reactions; introducing a co-reactant into the reaction vessel; and maintaining the inert gas flow, co-reactant flow and precursor supply for a time sufficient to make the phosphor particles moisture resistant.
The objects are further accomplished by the provision of a method of making moisture-resistant phosphors which comprises the steps of introducing an inert gas into a reaction vessel; charging phosphor particles into the reaction vessel; heating the reaction vessel to a reaction temperature; introducing a nitride coating precursor into the reaction vessel in a manner to avoid restrictive reactions; introducing a co-reactant into the reaction vessel; and maintaining the inert gas flow, co-reactant flow and precursor supply for a time sufficient to coat the phosphor particles.
The nitrided phosphor particles produced by this method had excellent efficacy ratings and strong luminance values in lamps after 100 hours use in high humidity (i.e.,  greater than 95%) and can be made in viable commercial quantities, such as 50 kg batches.